The present invention relates to an arrangement for correcting measurement errors produced by movement induced acceleration in a gravity referenced inclination sensor, or inclinometer. It is often desired to know the angle of inclination of a movable member of a machine with respect to the gravity of the earth or with respect to other machine elements. One method to accomplish this is to attach gravity referenced inclinometers to the movable member or elements. The present invention provides a means of improving the performance of such inclinometers during those times when the movable member rotates. The present invention corrects for the effects of tangential and rotational acceleration on inclinometer measurements made with inclinometers that sense acceleration due to gravity.
It is particularly useful to be able to monitor the orientation of various construction equipment elements. Such movable elements may, for example, be the boom, the dipper stick, the bucket or the chassis platform of an excavating machine, or the blade or chassis platform of a bulldozer, grader, or scraper. In many instances, the movable member pivots or rotates about a known point or axis as its inclination angle is changed under operator control, or under the control of an automated control system. Gravity referenced inclinometers are particularly suitable for use in construction equipment applications for a number of reasons, such as for example their ruggedness, reliability, small size, ease of installation, and low cost. The useful and desired output signal from such a gravity referenced inclinometer is a measurement of its inclination angle with respect to level. Level is measured with respect to the gravity of the earth. To measure incline angle with respect to gravity, the sensor responds to the vector component of gravitational acceleration that lies along a sensitive axis or axes. This type of sensor is inherently sensitive to acceleration produced by machine movement, since it cannot distinguish between gravitational acceleration and non-gravitational sources of acceleration. Linear, or translational, motions and vibrations also produce measurable accelerations. These non-gravitational sources of acceleration can result in undesired and erroneous output signals from an inclinometer. The magnitude of such erroneous signals is proportional to the non-gravitational acceleration experienced by the inclinometer. In some cases, the undesired signals can be large in comparison to the desired inclination measurement, rendering the sensor output unreliable and unusable until after cessation of the movements that caused this undesired acceleration.
When a movable member is pivoted about an axis, acceleration measurement errors in a radial direction will result from the rotation of the member. When the speed of this rotation changes, acceleration measurement errors will also result in a direction that is tangential to the direction of movement. This tangential acceleration is defined as the linear, or translational acceleration of a point on a rotating object at a distance “r” from the axis of rotation. Tangential acceleration at any location on the movable member is the product of the angular acceleration and the radial distance “r” from the pivot point or axis about which the movable member pivots or rotates. A gravity referenced inclinometer attached to a movable member of a machine might ideally be located at the axis about which that member pivots, or rotates. There, the distance “r” would equal zero, and there would be no tangential acceleration acting on the inclinometer from angular movement of the moving member. There are practical circumstances in which it is not possible or desirable to locate the inclinometer at the axis about which the member rotates.
Systems for displaying or controlling the position of movable elements on construction machines presently exist in a number of forms. Among the available sensors, gravity referenced inclinometers are useful in construction equipment applications for a number of reasons, including ruggedness, reliability, small size, ease of installation, and low cost. When such a gravity referenced inclinometer is not located precisely at the axis about which a movable element on which it is mounted pivots or rotates, it is difficult to obtain an accurate inclination reading. It has been common to filter the output of such an inclinometer to obtain a more stable signal with less jitter superimposed on it. However, low-pass filtering also introduces latency to the measurement of angle. As a result, the machine, or its elements, must be held stationary for a short time in order to obtain accurate position information. Additionally, latency is undesirable in a control system, potentially contributing to system instability.
It is seen, therefore, that there is a need for an arrangement to correct for measurement error induced in a gravity referenced inclinometer by acceleration when the inclinometer is mounted on a machine element that pivots or rotates, and for such an arrangement in which the speed and accuracy of the inclinometer are enhanced.